(AP) A Yemeni soldier stops a car at a checkpoint in a street leading to the U.S. embassy in Sanaa,...Full Image

WASHINGTON (AP) - The State Department on Tuesday ordered non-essential personnel at the U.S. Embassy in Yemen to leave the country following the threat by al-Qaida that has triggered temporary shutdowns of 19 American diplomatic posts across the Middle East and Africa.

The department said in a travel warning that it had ordered the evacuation of non-emergency U.S. government personnel from Yemen "due to the continued potential for terrorist attacks" and said U.S. citizens in Yemen should leave immediately because of an "extremely high" security threat level.

"As staff levels at the Embassy are restricted, our ability to assist U.S. citizens in an emergency and provide routine consular services remains limited and may be further constrained by the fluid security situation," the travel warning said. The U.S. Embassy is located in Sanaa, the capital of Yemen.

A U.S. intelligence official and a Mideast diplomat told The Associated Press that the current shutdown of embassies in the Middle East and Africa was instigated by an intercepted secret message between al-Qaida chief Ayman al-Zawahri and Nasser al-Wahishi, the leader of the Yemen-based al-Qaida in the Arabian Peninsula, about plans for a major terror attack. The officials spoke on condition of anonymity because they were not authorized to discuss the matter publicly.

The U.S. has pummeled terrorist leaders in Yemen with targeted drone strikes. On Tuesday, Yemeni security officials said a suspected U.S. drone killed four alleged al-Qaida members in a volatile eastern province of the country. The drone fired a missile at a car carrying the four men, setting it on fire and killing all of them, the officials said.

The Yemeni officials, speaking on condition of anonymity because they are not allowed to talk to the media, said they believe one of the dead is Saleh Jouti, a senior al-Qaida member. It's the fourth drone attack in the past week to hit a car believed to be carrying al-Qaida members.

The State Department on Sunday closed a total of 19 diplomatic posts until next Saturday. They include posts in Bangladesh and across North Africa and the Middle East as well as East Africa, including Madagascar, Burundi, Rwanda and Mauritius.

Jen Psaki, spokeswoman for the State Department, said in a separate statement issued early Tuesday that the department issued the order for Yemen because of concern about a "threat stream indicating the potential for terrorist attacks against U.S. persons or facilities overseas, especially emanating from the Arabian Peninsula."

The statement said U.S. citizens who choose to stay in Yemen despite the travel warning should limit nonessential travel within the country and make their own contingency emergency plans. "From time to time, the Embassy may temporarily close or suspend public services for security reasons," Psaki said. "Emergency assistance to U.S. citizens during non-business hours (or when public access is restricted) is available through embassy duty personnel."

Britain's Foreign Office also announced that it had evacuated all staff from its embassy in Yemen due to security concerns. The office said the British embassy staff were "temporarily withdrawn to the U.K." on Tuesday, but declined further comment. Previously, the U.K. had said the embassy would be closed until the end of the Muslim festival of Eid later this week.

AQAP, gathered in small cells scattered across Yemen's vast under-governed regions, has proven to be a tenacious enemy.

Officials say al-Zawahri, who took over for Osama bin Laden and works from Pakistan, has reached out to the Yemeni branch, cementing their ties and further signaling the AQAP is once again looking to target U.S. and Western interests after a sustained period of more local and regional focus.

For puzzled Americans who've been told that al-Qaida is on the decline, the latest warnings raise questions about how successful America's war on terror has been and whether the terror group has been able to reorganize and reconstitute itself since bin Laden's death in May 2011.

And, although U.S. officials agreed a year ago to restart military aid to Yemen, it's unclear how much of the new aircraft and weapons have arrived. After aid to Yemen was frozen for some time, the U.S. military is once again on the ground there training Yemeni special operations forces and has delivered more than a dozen helicopters to the Yemeni military and provided training for them, U.S. defense officials said.

But other weapons and equipment are still in the pipeline, according to a Mideast official.

The latest terror alert was triggered in part when the secret message between al-Zawahri and al-Wahishi was intercepted several weeks ago.

There long has been movement of fighters between Pakistan and Yemen, and discussions between the two groups, but the latest communication triggered worries and prompted the U.S. to take steps to boost security. The embassy closures came one day after a meeting between President Barack Obama and Yemeni President Abdo Rabu Mansour Hadi.

AQAP has been widely considered al-Qaida's most dangerous affiliate for several years. Even though the group lost Anwar al-Awlaki - one of its key inspirational leaders - to a U.S. drone strike in 2011, al-Wahishi and the group's master bomb maker, Ibrahim Hassan al-Asiri, remain on the loose and determined to target the U.S. and other Western interests.

The group is linked to the botched Christmas Day 2009 bombing of an airliner bound for Detroit and explosives-laden parcels intercepted aboard cargo flights a year later - both incidents involving al-Asiri's expertise.

In recent years, however, AQAP has been focused more on making gains at home, taking advantage of an unstable government and overstretched military that was forced to concentrate on protecting the political center in Sanaa. As a result, said a senior defense official, AQAP was able to expand its foothold in the south, capture more weapons and gain control of additional territory.

Obama and others have consistently alluded to the weakening of core al-Qaida in Pakistan - particularly since a Navy SEAL team killed bin Laden in Pakistan two years ago. Obama frequently touts bin Laden's death in his speeches, and has declared that the terror group was "on the path to defeat."

But while core al-Qaida may be on the ropes, officials have warned repeatedly that its offshoots in places like Yemen and Africa, as well as homegrown believers in the U.S., have grown increasingly dangerous and more difficult to track.

On Monday, officials declined to be more specific about the latest threat.

"What we know is the threat emanates from, and may be focused on, occurring in the Arabian Peninsula," White House spokesman Jay Carney said. "It could potentially be beyond that, or elsewhere."